Have you ever wondered how flatworms— you know, those billowing tissue-like invertebrates — reproduce? Well, certain Platyhelminthes engage in a behavior known as "penis fencing," which involves two hermaphroditic creatures dancing a bizarre sexual tarantella in hopes of impregnating their opponents. Explains PBS of this strange duel:

During penis fencing, each flatworm tries to pierce the skin of the other using one of its penises. The first to succeed becomes the de facto male, delivering its sperm into the other, the de facto female. For the flatworms, this contest is serious business. Mating is a fight because the worm that assumes the female role then must expend considerable energy caring for the developing eggs.

In other words, it's kind of like West Side Story (if the Sharks and the Jets made love onstage after every song). You can watch Sue Morrison of the Western Australian Museum narrate the mating ritual above, and here's another handsome couple dirty dancing sans commentary.